"This is a tragedy," said Dr. William Schaffner, chairman of preventative medicine at Vanderbilt University Medical Center.

Only six months into the calendar year, 152 Americans have already been diagnosed with measles ? double the average number of cases in a half-year, and the largest outbreak in nearly two decades, according to the Centers for Disease Control and Prevention.

Thirty-five percent of the patients were hospitalized, and nine cases progressed to pneumonia, according to the CDC.

Signs of measles, a highly contagious viral respiratory disease, can easily be confused with the common cold. The disease progresses quickly and can lead to fatal complications.

Most who were diagnosed were not vaccinated against the disease, the CDC said. The combination measles, mumps, and rubella vaccine is the strongest protection not only those who get the shot, but also those who are immunocompromised, thus vulnerable to the disease and unable to receive the vaccine, said Dr. Larry Givner, chief of pediatric infectious diseases at Wake Forest Baptist Medical Center.

The first dose of the combination MMR vaccine is given to children 12 to 15 months old, according to CDC recommendations. The second dose is usually given before the start of kindergarten to ensure that a child has developed immunity.

The MMR vaccine has contributed to a 99 percent reduction in cases in the U.S.

"It highlights the importance of having vaccinated against common diseases, not just measles," said Givner.

The spike in reported measles cases comes at the heels of a survey released in early June by the CDC and the U.S. Department of Health and Human Services National Vaccine Program Office that showed that nearly 80 percent of parents are uncomfortable about having their children vaccinated in general, not just against the measles.

Young parents have forgotten about measles because they have moms who didn't experience it and they haven't been told," said Dr. Schaffner.

While many of the parents are simply queasy about the needle, their concerns include doubts about the safety of vaccines, the number of vaccines their children receive, and the persistent worries that the vaccines are to blamed for disabilities like autism, the study said.

The MMR vaccine has been caught in the crossfire of decade-long debates over whether the vaccine can lead to autism and other developmental disorders in children.

Although the theory has been widely discredited, many parents continue to receive conflicting messages regarding its safety.

"We should be weary of those who try to give medical advice but are not experts, especially those who are famous but not for their vaccine work," said Givner.

Concerns of fevers resulting from the vaccines were mentioned by 32 percent, and the fear that the vaccines could cause learning disabilities like autism remains among 30 percent. Others, 26 percent, have a general worry that the ingredients in the vaccines are unsafe, the survey found.

Despite their concerns, at least 95 percent of parents have their children vaccinated while only 5 percent skip some of the vaccines. Only 2 percent say they would avoid vaccines altogether.

Parental uncertainty over vaccinations means there is a continuing need for reassurance and education about the necessity of getting kids immunized, the CDC said.